Variable pitch propeller systems typically incorporate a plurality of propeller blades mounted to a rotary hub driven by the aircraft's engine, with each propeller blade extending radially outwardly from the hub along the longitudinal axis of the blade. In order to permit pitch adjustment, each blade is mounted to the hub for pivotable movement about its longitudinal axis. The hub typically encloses a chamber within its interior wherein a pitch change actuation system is disposed in operative association with the propeller blades. The actuation system functions to selectively change the pitch of the blades thereby altering air resistance to the rotation of the blades to thereby control engine speed.
In most modern aircraft, the pitch change actuator is of the hydromechanical type wherein an output member, typically a piston, is driven in response to adjustments in the pressure of the hydraulic fluid which drives the actuator. The adjustments in fluid pressure are typically affected by either a hydromechanical or electronic control system which monitors engine speed and causes, by way of collateral apparatus, a change in fluid pressure whenever the monitored engine speed departs from the desired engine speed setting.
Such hydromechanical pitch change actuation systems are well known in the art. For example, commonly assigned U.S. Pat. No. 4,523,891 to Schwartz and Duchesneau discloses a conventional pitch actuation system wherein each propeller blade is operatively connected to a piston which is driven by the pressure of a fluid which is selectively directed in response to a departure from desired engine speed against the opposite faces of the piston thereby causing a linear displacement of the piston and a resultant change in pitch of the blades operatively connected to the piston. The piston is reciprocally moveable within a cylinder disposed within the hub about a torque tube which extends from the fluid supply to the piston. As shown in U.S. Pat. No. 4,523,891, the pressure fluid is conveyed through a conduit within the torque tube from the fluid supply to a valve associated with the piston which, depending upon its position, selectively directs the fluid against either the front or the rear face of the piston thereby causing the piston to move linearly thereby rotating the blade or blades associated therewith to effectuate the change in pitch. The position of the valve directing the fluid to this piston is controlled by the rotation of a ball screw operatively connected to the longitudinally opposite end of the torque tube, typically by a quill and spline arrangement, such that the torque tube rotates with the ball screw. Rotation of the torque tube results in a corresponding rotation of a of second screw, termed a pitch lock screw, mounted to the valve, which in turn causes the valve to linearly translate in a longitudinal direction so as to selectively register the valve relative to the piston. The system is designed to safeguard against inadvertent pitch reduction by failing operational at the last commanded pitch angle in the event of a loss in fluid pressure thereby avoiding excessive pitch fining and an undesirable increase in drag. The drawback to this type of system is its complexity, and the inherent backlash associated therewith and its effect on synchrophasing accuracy
In an alternative prior art pitch change actuation system, two independent conduits may be provided within the an elongated non-rotating tube with one of the conduits directing fluid from the supply to the front face of the piston and the other conduits directing fluid from the supply to the rear face of the piston. In such a case, a valve system meters fluid from the supply selectively between the two conduits so as to cause the correct directional movement of the piston. The ball screw and pitch screw arrangement of the system disclosed in U.S. Pat. No. 4,523,891 is not utilized in this type of system. Due to the lack of a pitch lock screw, a counterweight is provided on the blade shaft which prevents a reduction in blade pitch in the event of inadequate fluid pressure. Although much less complex, this system is costly as the counterweights add weight to the primary structure of the aircraft and reduce fuel economy.
Accordingly, it is an object of the present invention to provide a pitch change actuation system which exhibits little or no backlash to interfere with synchrophasing accuracy.
It is a further object of the present invention to provide such a pitch change actuation system having a lower overall weight.